1. Field of Invention
This invention relates to control mechanisms and particularly to those which perform the control function by selectively positioning a piston within a cylinder and with the piston rod extending through the cylinder wall so that its free end produces the control motion. Such controls are conventionally used on aircraft and other vehicles in which jamming of the piston or piston rod within the cylinder produces control malfunction. One possible cause of piston or piston rod jamming within a cylinder is the striking of the control by a projectile in flight wherein the ballistic damage caused by such a projectile in passing through the control cylinder may displace the cylinder material so as to impede piston motion or cause piston jamming, and may also pass through the piston rod so that the displaced piston rod material cannot pass through the aperture provided in the cylinder for that purpose, thereby also either eliminating piston motion or causing piston rod and hence piston jamming.
2. Description of the Prior Art
In the flight control art, attempts have been made in the past to reduce or eliminate the ballistic vulnerability of a flight control by fabricating the cylinder and/or piston as armor utilizing conventional armor-plate techniques. Such armor construction is shown in Sliney U.S. Pat. No. 3,566,741 which teaches tubular armor plate formed from an impact-resistant outer armor shell and a slightly softer inner armor shell. Such constructions proved to be unreasonably heavy and large, and therefore, unacceptable for aircraft use and, further, they proved not to provide the desired ballistic-proof result.
Simmons U.S. Pat. No. 3,884,127 teaches such a control in which both the piston and the cylinder gland through which the piston rod extends are fabricated so as to be shearable so as to prevent piston or piston rod jamming. The Simmons patent has the disadvantage, however, that such flight controls are required to be tested to demonstrate that their structural integrity is such that they can withstand without rupture or permanent deformation 2.5 times the normal operating pressure which causes the piston to reciprocate within the cylinder. This test requirement dictates that the area of the piston must be at least 21/2 times the area of the cylinder gland to be able to demonstrate such structural integrity and still result in a cylinder gland which is shearable under normal operating conditions. This requirement that the piston area be at least 21/2 times the gland area results in a control of larger envelope and a greater weight that would otherwise be necessary. This envelope and weight problem is added to by the fact that larger hydraulic chambers formed by such a piston require the use of more operating fluid, with the attendant requirement for a larger fluid supply and dispensing system. In addition, such a large piston generates larger-than-normal operating forces which must be reacted by the remainder of the control system so that the remainder of the control system must be correspondingly strengthened, further increasing the weight problem.